Under ordinary circumstances, the thermoregulatory system of the human body maintains a near constant temperature of about 37° C. (98.6° F.), a temperature referred to as normothermia.
For various reasons, however, a person may develop a body temperature that is below normothermia, a condition known as hypothermia, or a temperature that is above normothermia, a condition known as hyperthermia. Hypothermia and hyperthermia are generally harmful, and if severe, the patient is generally treated to reverse the condition and return the patient to normothermia. Accidental hypothermia significant enough to require treatment may occur in patients exposed to overwhelming cold stress in the environment or whose thermoregulatory ability has been lessened due to injury, illness or anesthesia. For example, this type of hypothermia sometimes occurs in patients suffering from trauma or as a complication in patients undergoing surgery. Likewise, examples of hyperthermia include exposure to overwhelming exposure to hot environmental stimulation, injury or illness, or complications of anesthesia.
However, in certain other situations hyperthermia and especially hypothermia may be desirable and may even be intentionally induced. For instance, hypothermia is generally recognized as being neuroprotective, and may, therefore, be induced in conjunction with treatments for ischemic or hemorrhagic stroke, blood deprivation such as caused by cardiac arrest, intracerebral or intracranial hemorrhage, and head and spinal trauma. In each of these instances, damage to neural tissue may occur because of ischemia, increased intracranial pressure, edema or other processes, often resulting in a loss of cerebral function and permanent neurological deficits.
Other examples where hypothermia may be neuroprotective include periods of cardiac arrest in myocardial infarction and heart surgery, neurosurgical procedures such as aneurysm repair surgeries, endovascular aneurysm repair procedures, spinal surgeries, procedures where the patient is at risk for brain, cardiac or spinal ischemia such as beating heart by-pass surgery or any surgery where the blood supply to the heart, brain or spinal cord may be temporarily interrupted.
Hypothermia has also been found to be advantageous as a treatment to protect both neural tissue and cardiac muscle tissue during or after a myocardial infract (MI).
Simple surface methods for warming, whether treating accidental hypothermia or reversing hypothermia, include wrapping the patient in warming blankets or immersing the patient in a warm water bath. If the hypothermia is not too severe, and the need to reverse the hypothermia is not to urgent, these methods may be sufficient. However, normal thermoregulatory responses such as vasoconstriction of capillary beds at the surface of the body and arterio-venous shunting of blood away from the skin act to make the surface application of warmth very slow and inefficient.
Where hypothermia is desired, for example where a patient has suffered a stroke, an attempt may be made to cool the patient by application of cooling blankets or alcohol rubs. These attempts to induce hypothermia by application of surface cooling are slow and inefficient since the application of a cold blanket or cold alcohol to the patient's skin will triggers these thermoregulatory responses. Furthermore, surface cooling will be extremely uncomfortable for any awake patient. As an additional confounding factor, the patient is often induced to shiver, greatly increasing the amount of heat generated by the body, perhaps by a factor of five or more. This is and often sufficient to make further reduction of patient temperature impossible. Shivering also markedly increases the discomfort of the patient, sometimes to the degree that continued surface cooling is not possible.
Sometimes warmed or cooled breathing gases or warm infusions are applied to heat or cool a patient. These are also slow and poorly controlled since the amount of heat that can be added or removed is limited by the amount of infusate and the limitations on the temperature of the gas or infusate that can be used without harming the patient.
A very invasive method of controlling the temperature of a patient is sometimes employed in which the patient's blood is shunted through a cannula (attached to a vein such as the inferior vena cava) to an external pump, and then pumped back into the patient's body. The blood removed from the patient is heated or cooled externally before it is reintroduced into the patient's body. An example of such a by-pass arrangement is the Cardio-Pulmonary By-pass system (CPB) often used in open heart surgery. This by-pass method, once it is initiated, is both fast and effective in adding or removing heat from a patient's blood and in exercising control over the patient's body temperature in general, but has the disadvantage of involving a very invasive medical procedure which requires the use of complex equipment, a team of highly skilled operators, is generally only available in a surgical setting, and because of these complexities, is extremely expensive and requires a long time to initiate. In fact, it generally cannot begin until after the patient's thorax has been surgically opened, cannot re-warm after the thorax is closed. Furthermore, such a by-pass method also involves mechanical pumping of blood, which is generally very destructive to the blood resulting for example in hemolysis. For this last reason, use of by-pass for more than four hours is generally considered undesirable which limits the use of this technique for lengthy temperature control.
Another method for adding or removing heat that does not involve pumping the blood with an external, mechanical pump include a method of treating or inducing hypothermia or hyperthermia by means of a heat exchange catheter placed in the bloodstream of a patient was described in U.S. Pat. No. 5,486,208 to Ginsburg, the complete disclosure of which is incorporated herein by reference. The Ginsburg patent discloses a method of controlling the temperature of a body by adding or removing heat to the blood by inserting a heat exchange catheter having a heat exchange region into the vascular system and exchanging heat between the heat exchange region and the blood to affect the temperature of a patient. One method disclosed for doing so includes inserting a catheter having a heat exchange region comprising a balloon into the vasculature of a patient and circulating warm or cold heat exchange fluid through the balloon while the balloon is in contact with the blood.
In general, the transfer of heat between the heat exchange region of such a catheter and the flowing blood may be expressed by the following formula:Q=USΔTwhere Q is the heat transfer rate in watts, U is overall heat transfer coefficient, S is the surface area of the interface between the heat exchange region and the flowing blood, and ΔT is the temperature difference between the flowing blood and the heat exchange region. In order to maximize the speed and control of heat transfer between the heat exchange catheter described below and the blood of a patient, the heat transfer rate (Q) must be maximized.
However, for a given heat exchange catheter, for example the catheter described in the Detailed Description below, the surface area of contact between the heat exchange region and the blood (S) is fixed.
For heat exchange between the catheter and the blood, the ΔT is also limited. The blood is generally the same temperature of the body, between 37° C. and 32° C. The heat exchange region cannot be maintained much below 0° C. or it will freeze the blood in contact with the heat exchange region. Any temperature above about 50° C. is generally considered harmful to the blood, so the heat exchange region generally will not be maintained at a temperature significantly below 0° C. or above 50° C.
The variable U is determined by a number of different variables including the material of the heat exchange catheter, the material of the flowing fluid (blood) the rate of flow, and the like. For a given heat exchange catheter in the blood, the material of the catheter is fixed, and the heat exchange nature of the blood and the rate of blood flow are generally not within the control of the physician. With all these parameters fixed or limited, it would be extremely advantageous to devise a way to further enhance heat transfer where the above conditions exist. This invention makes possible such an enhancement.